Long pause, followed by a raise (very strong):
This has to be the cornerstone tell of most players. The irony is that by using this strategy, they are actually giving off their own tell. The whole logic behind this play screams strength, because people usually only do it when there is a big possible hand on the table, like a flush or straight. You bet, your opponent pauses for about 10-15 seconds and then raises you. In English, he might as well be saying: "Hmm... there's a flush possibility on the table and you've bet into it. Man, I really have to think about this because you might have a really strong hand! Oh well, I think I'm beat, but I *guess* I'll raise...". Don't fall for this. Sometimes, a player legitimately thinks you're bluffing the river or whatever so he's raising you back, but most of the time he's got a big hand.
Instant automatic raise (usually very strong):
By automatic raise, I mean a raise that is so fast that it can only happen when a user has checked the 'bet/raise all' button. While occasionally this is used by maniacs and bluffers, most of the time it's used by a player who is so sure of his hand that he is willing to bet or raise regardless of the action. This usually means that you want to tread carefully and consider folding or check-calling this down. It should always raise a big warning sign at the very least.
Instant automatic check (weak or folding hand):
This is a pretty straight-forward tell, as almost every player uses the auto-check button only when they are willing to fold their hand. Sometimes players will be willing to call after auto-checking, but it's incredibly rare to see someone check-raise with the auto-check button. So, when you're in a small field and it's auto-checked to you, you can often take down the pot right then and there.
Odd number bet, such as betting $9.95 instead of $10 (neutral hand):
This comes up quite often on the internet no limit games, where players are able to bet with fractions of a dollar. When a player does this, you shouldn't look into it either way. Some players do this routinely to throw off their opponents, others do this to make their bet size look bigger and scarier (induce folds), while others try to get you to call when doing this.
Opponent quickly calls your bet (moderate to semi-weak hand):
Usually when someone is chasing you or has a marginal hand, they'll make a fast call to look as if they're not scared of your bet. With reverse psychology, you can figure out that this is an intimidation ploy, because if they did have a strong hand, they'd be raising your bet instead of flat calling. In this situation, you can often continue to bet as normal. That said, some players on tilt or experienced players will often make quick calls, as they really do intend on calling to the river